A conventional inductive proximity sensor of that type includes a coil in an oscillator circuit, generating a magnetic field that emanates from a sensing face of the sensor housing into a desired sensing area. Movements of an external object inside the sensing area or into the sensing area can be detected by the sensor circuit based on variations of the magnetic field caused by the object. More precisely, the magnetic energy absorbed by the object and/or interactions of the emanating magnetic field with a magnetic field caused by eddy currents generated in the object are exploited for object detection.
An alternate version of such an inductive proximity is disclosed in EP-A 0 492 029. The proximity sensor includes a coil that is supplied with periodic current pulses to generate a magnetic field that emanates into the sensing area. Due to the emanating magnetic field, voltages can be induced in an external object which provoke a decaying current in the object after the end of a periodic current pulse in the proximity sensor. The decaying current in the external object in turn induces a voltage in the coil of the proximity sensor after the respective periodic current pulse. Corresponding measurement signals of these induced voltages allow a detection of movements of the external object inside the sensing area or into the sensing area.
The widespread usage of these sensors for various sensing functions, in particular in connection with industrial plants or machinery such as robot systems, assembly systems, material handling systems, etc., makes it desirable to achieve a better performance and reliability with respect to various device characteristics including temperature resistance, contamination protection and lifetime cycle.
It is therefore an object of the present invention to improve at least one of the above mentioned deficiencies and to provide the initially addressed proximity sensor so as to provide a higher resistance against harmful external influences, such as high temperature and contamination, and/or to prolong its life cycle and reliability.